The serial data line is the means by which the microprocessor-controlled modules in the vehicle communicate with each other. Each module is assigned a specific recognition code known as source ID. This code is used to identify which module is communicating on the serial data circuit. When a message is sent out on the serial data circuit , the recognition code will identify which particular module sent the message. These messages contain specific information, or commands, modules require to function properly. When a module receives a message it also "learns" the source ID of the module that sent it. This source ID is used to determine which module actually sent the message and for state of health (SOH) monitoring. When the ignition switch is in RUN, each module communicating on the class 2 serial data line sends a SOH message every 2 seconds to ensure that the module is operating properly. When a module stops communicating on the class 2 serial data line, for example if the module loses power or ground, the SOH message it normally sends on the data line every 2 seconds disappears. Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its absence. Those modules expecting the source ID then set an internal DTC associated with the loss of SOH of the non-communicating module. The DTC is unique to the module which is not communicating. For example, when the body control module (BCM) SOH message disappears, several modules set DTC U1064. Note that a loss of serial data DTC does not normally represent a failure of the module that set it. Description and diagnosis of each module controlled system will be covered in the respective service manual section.

The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. The DLC is a standardized 16 way connector located below the instrument panel and close to the steering column. Both the connector design and location are dictated by industry wide agreements. All DLCs are required to provide a power supply, hot at all times, to terminal 16. This circuit is used to power the scan tool. All DLCs supply a power ground at terminal 4 and a signal ground at terminal 5. The class 2 serial data circuit is provided at terminal 2 of the DLC.

Once the scan tool is connected to the class 2 serial data link through the DLC, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes.

The class  2 serial data link allows these modules to communicate data with each other:

In addition, the class 2 serial data link allows a scan tool to communicate with these modules for diagnostic and testing purposes.